With the ASUS O!play, when you plug the stick in and it mounts, it can take up to 30 seconds for the FAT32 selection to become available.  Please just be patient when selecting the USB drive.  The ASUS device seems to need to count all the files and the drive size before it will allow you to select it.  Please allow up to 30 seconds for the drive to become available for browsing when selecting the FAT32 storage option.